In FIG. 1, a conventional index mechanism is shown and is a one indexing-type roller gear cam mechanism which utilizes cylindrical rollers 10 radially arranged on the turret and one or more ribs 11 on a globoidal cam 13. A linear rotation on the input axis of the cam generates intermittent and indexing motion on the output axis of the turret 12. In some applications, the automatic tool changer (ATC) of a CNC machine employs the roller gear cam of a spatial cam mechanism to change cutting tools. This kind of mechanism can also be utilized in an automatic feed apparatus and in various other automatic manufacturing devices.
When the axis of the globoidal cam rotates at constant speed, some of the rollers radially arranged on the turret will engage the side-wall of the rib. The linear rotation on the input axis will therefore be transferred into a dwell or an index motion on the output axis of the turret. If the rib is straight i.e., straight with reference to a base circle defined around the cam axis), the turret will remain stationary. This is called the dwell period. A mechanism fixed on the output axis can, for example, finish a necessary machining operation within the dwell period. After the dwell period, if the rib is helical, the engaged rollers travels along the rib and makes the turret rotate a predetermine angle. This is called the index period. Within the index period, the mechanism fixed on the output axis is rotated to the next dwell position and then the next or the same machining operation can be performed. When a roller leaves the rib, another one comes into contact with that or a different rib, i.e., it is meshed with the rib. This is the operating sequence of the roller gear cam index mechanism.
In order to ensure a better meshing condition between all of the presently engaged rollers and the ribs, yield an accurate transmission of motion, and decreases the wear on the contact surfaces of the engaged rollers and ribs, the roller gear cam index mechanism usually has an eccentric ring on the input axis of the globoidal cam for adjustable assembly of the mechanism. Thus, the distance between two central axes of the mechanism can be adjusted when it is assembled. A better contact condition between the engaged rollers and ribs can be obtained. Meanwhile, the eccentric ring also is used in adjusting the suitable preload to avoid separation or extreme contact stress between the engaged rollers and ribs during actual use.
In the conventional roller gear cam index mechanism, which has been well known and used, there still exists some inherent shortcomings:
1. The conventional mechanism is not highly accurate or stable.
When any particular roller travels between two adjacent ribs, two ribs will be alternately disposed among three rollers. The second (middle) roller will be simultaneously jammed into the side-walls of two adjacent ribs, as shown in FIG. 1. In order to avoid such a jamming effect, the second roller must only touch a side-wall of one of the ribs or not at all.
However, this reduces the accuracy and stability of the mechanism.
2. The second roller contacts one side-wall of one rib and then another, causing unexpected wear. When any engaged roller travels from one side-wall of a rib to another when between two adjacent ribs, the rotational direction of this roller reverses. Because the roller gear cam index mechanism operates at high speed, the reversed rotation of the roller quickly causes wear between the roller and the rib. This decreases the life of the mechanism.
3. There is no guarantee that the meshing condition in each index period can be correctly adjusted.
The ideal contact condition between one engaged cylindrical roller and the rib is line contact. Some manufacture and assembly errors, however, will make the intended line contact become edge contact between the engaged rollers and the ribs. Because just the edges of the engaged rollers and the ribs are in contact noise, vibration, and extremely high contact stress in these contact regions results which will decrease the life of the mechanism. In order to avoid this ill contact condition, an eccentric ring is used to adjust the contact condition between the engaged rollers and the ribs. However, a better contact condition can be only determined in this manner when the turret rotates to a given position. Since each roller on the turret will contact the same rib in series, the contact conditions between these other rollers and ribs can not be guaranteed. Thus by using the eccentric ring, the meshing condition in each index period can not be correctly adjusted at the same time. In addition, another function of the eccentric ring is to adjust the preload condition. This function conflicts with that of setting contact conditions because it is hard to adjust the contact condition and the preload condition by using a same eccentric ring at the same time.
4. The preload condition between the turret and the globoidal cam in each index period can not be suitably adjusted.
In order to ensure proper preload condition between the turret and the globoidal cam, the distance between two rotation axes of the said mechanism can be adjusted to provide suitable preload by an eccentric ring on the output axis. The suitable preload, however, can be only adjusted when the turret rotates to a given position. Because of manufacturing and assembly errors, the preload condition between the turret and the globoidal cam in each indexing period can not be suitably adjusted. Thus, a comprised distance between the two rotation axes of the mechanism is used to provide better preload for all indexing periods. Because the relative distance is not the same as that which was originally designed, the accuracy of this mechanism is decreased. Further, as was mentioned in part 3, another function of the eccentric ring is to adjust the contact conditions between all engaged rollers and ribs. These two functions conflict with each other because it is hard to adjust the contact condition and the preload condition by using the same eccentric ring at the same time.
Tetusya Oizumi & Co. at the ASME conference in 1992 submitted the globoidal-cam worm gearing transmission assemblage. The roller-wheel of the worm gearing transmission assemblage enables more rollers to be installed. This could achieve the goal of adjusting the preload condition. Although the globoidal-cam has the benefit of no backlash, the objective for using the rollers is to replace the tooth surfaces of the worm gear. The said cam changes the phase angle between the upper and lower roller wheels to provide the preload. A set of rollers on each wheel will overlap and become a pair of roller wheels. When adjusting the preload between the engaged rollers and the ribs, the position of the central axis of each roller has been changed. The meshing condition between each engaged roller and rib is not the same as the ideal condition.
Robert J. Petroff & Co in 1969 obtained U.S. Pat. No. 34,565,929. In their patent claim, they changed the curvatures of the rollers and ribs to change the meshing condition from the original line contact to point contact. Because the radius and the way of line up are not changed, the above four mentioned defects can not be avoided.